In a small bowl stir water and cornstarch until combined. Halve and pit plums. In a 12-inch skillet cook plums and sugar over moderate heat, stirring, until sugar is melted. Stir in cornstarch mixture and simmer, stirring, 15 minutes, or until mixture is thickened. Transfer plum mixture to a 15 x 9-inch (3-quart) baking dish.

Sprinkle topping over plums. Bake mixture in middle of oven 40 to 45 minutes, or until topping is crisp and golden, and cool on a rack 10 minutes.